I've been asked this by my children's pediatrician as well as my endocrinologist. With my children's doc, I just replied that I refuse to answer the question. I was pressed by a nurse once and simply told her it was none of their business.
My endocrinologist asked me this on a patient information sheet. I just wrote a note that said "I fail to see how this pertains to the treatment of my thyroid."
Good for you. It's none of their business.